1. Field of the Invention
The present invention is directed to a method for measuring the dose distribution in a computed tomography apparatus, wherein the dose distribution of an X-ray beam emitted by an X-ray tube with a thickness prescribed by a radiation diaphragm is acquired in the thickness direction for at least one setting of the radiation diaphragm.
2. Description of the Prior Art
A computed tomography apparatus has, among other things, an X-ray tube, an X-ray detector with one or more lines and a patient table. The X-ray tube and the X-ray detectors are arranged at a gantry that rotates around the patient table or an examination axis that proceeds parallel thereto. As an alternative, the X-ray detectors can be arranged around the patient table on a stationary detector ring, with only the X-ray tube moving with the gantry.
The patient table is usually displaceable along the examination axis relative to the gantry. The X-ray tube generates a ray bundle that is spread fan-shaped in a slice plane perpendicular to the examination axis. This limitation of this ray bundle in the direction of the slice thickness is set by the size or diameter of the focus on the target material of the X-ray tube and by one or more radiation diaphragms arranged in the beam path of the X-ray bundle. These radiation diaphragms can have an adjustable aperture width for generating different thicknesses of the X-ray bundle. Further, radiation diaphragms having diaphragm apertures of different widths arranged next to one another are known that can be shifted to different positions in the slice thickness direction by means of a stepping motor, in order to set different thicknesses of the X-ray bundle. During an examination, the X-ray bundle penetrates a slice of a subject, for example a body slice of a patient, placed on the patient table, and strikes the X-ray detectors lying opposite the X-ray tube. The angle at which the X-ray bundle penetrates the body slice of the patient, and possibly the position of the patient table relative to the gantry, change continuously during the image acquisition with the computed tomography apparatus.
The intensity of the X-rays of the X-ray beam that strike the X-ray detectors after penetrating the patient is dependent on the attenuation of the X-rays by the patient. Dependent on the intensity of the received X-rays, each of the X-ray detectors generates a voltage signal that corresponds to a measurement of the overall transparency of the body for X-rays proceeding from the X-ray tube to the corresponding X-ray detector. A set of voltage signals of the X-ray detectors that correspond to attenuation data acquired for a specific position of the X-ray source relative to the patient is referred to as a projection. A set of projections that were acquired at different positions of the gantry during the revolution of the gantry around the patient is referred to as a scan. The computed tomography apparatus acquires many projections at different positions of the X-ray source relative to the body of the patient in order to reconstruct an image that corresponds to a two-dimensional tomogram of the body of the patient or to a three-dimensional image. The standard method for the reconstruction of a tomogram from acquired attenuation data is known as the method of filtered back-projection.
Typical slice thicknesses that are acquired with a computed tomography apparatus in a projection or during a scan lie in the range between 1 and 10 mm. The desired slice thicknesses can be set by the radiation diaphragm at the tube side. An important quality feature of a computed tomography apparatus is thereby the precision with which the thickness of the X-ray bundle that has been set and that corresponds to the slice thickness, is in fact achieved. Conventionally, a portion of an X-ray film in the beam path of the X-ray bundle for each collimation or slice thickness achievable by adjustment of the radiation diaphragm has been irradiated for quality assurance, particularly before the delivery of a computed tomography apparatus or after maintenance or repair work. The film is successively displaced by a defined distance and then irradiated with the next collimation. After developing the film, the width of the blackening generated with the X-ray bundle is measured as a criterion for the dose width. The precision with which the thickness of the X-ray bundle that has been set is in fact achieved can be recognized on the basis of this measured width.
This conventional procedure for quality assurance, however, is complicated and requires manual, and thus error-prone, evaluation of the film.
U.S. Pat. No. 6,134,292 discloses a method and an apparatus for the correction of image data with regard to an inhomogeneous distribution of the X-rays as well as with regard to the detector sensitivity of a computed tomography apparatus in the z-direction. In this method, the dose distribution of an X-ray bundle emitted by the X-ray tube having a thickness prescribed by a radiation diaphragm is acquired in the thickness direction by a setting of the radiation diaphragm in order to obtain a dose profile in the z-direction, on the basis of which the image data can be corrected. The publication, however, is not concerned with the aforementioned problem.